In the use of valves for controlling the pressure in hydraulic pipes, under certain conditions, when the valve discharges, resonance phenomena occur between the flow in the pipe and the moving part of the valve, causing that part to vibrate.
That circumstance does not affect the safety of the pipe but, by the shock effect of the shutter on its seat, the quality of the fluid-tight sealing can deteriorate, inasmuch as the seat is blocked rigidly in the body, which is itself fixed on the pipe and since the seat has no possibility of retracting.